CVE-2020-26959 in Firefox
Summary
by MITRE • 12/09/2020
During browser shutdown, reference decrementing could have occured on a previously freed object, resulting in a use-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 12/14/2020
This vulnerability represents a critical use-after-free condition that occurs during the browser shutdown process in Mozilla Firefox and Thunderbird applications. The flaw manifests when the browser attempts to decrement reference counts on objects that have already been freed from memory, creating a scenario where subsequent memory access operations can target deallocated memory regions. Such memory corruption vulnerabilities are particularly dangerous because they can be exploited to execute arbitrary code or cause application crashes that may be leveraged for privilege escalation attacks.
The technical implementation of this vulnerability stems from improper memory management during the browser's cleanup phase. When Firefox or Thunderbird terminates, the garbage collector or reference counting mechanism attempts to release resources that may have already been destroyed through previous cleanup operations. This creates a race condition where reference decrement operations target objects that no longer exist in the heap, leading to memory corruption that can be manipulated by malicious actors. The vulnerability specifically affects versions prior to Firefox 83 and Firefox ESR 78.5, as well as Thunderbird versions before 78.5, indicating that the issue was present in the reference counting implementation across these browser versions.
From an operational perspective, this vulnerability presents significant risks to end users and organizations that rely on these applications for daily operations. Attackers can potentially exploit this condition to execute remote code execution by crafting malicious web content or email messages that trigger the vulnerable shutdown sequence. The memory corruption could lead to application crashes that may be chained with other exploits to achieve privilege escalation or persistent access to affected systems. According to the CWE database, this vulnerability maps to CWE-416 which describes the use of freed memory condition, and aligns with ATT&CK technique T1059.007 for command and scripting interpreter usage in exploitation scenarios.
The impact extends beyond simple application instability as this vulnerability can be leveraged in various attack vectors including drive-by downloads, phishing campaigns, or malicious email attachments. Security researchers have noted that the exploitability of such use-after-free conditions is particularly high when they occur in browser contexts where attackers can control memory layout through crafted inputs. Organizations should prioritize immediate patching of affected versions to prevent potential exploitation, as the vulnerability's nature makes it suitable for automated exploitation tools. The recommended mitigation strategy includes updating to the patched versions of Firefox, Firefox ESR, and Thunderbird, while security teams should monitor for any reported exploitation attempts in the wild.